Influenza virus types A and B are members of the orthomyxoviridae family of viruses that cause influenza infection. The infective potential of influenza is frequently underestimated and can result in high morbidity and mortality rates, especially in elderly persons and in high-risk patients, such as the very young and immuno-compromised. Influenza A and B viruses primarily infect the nasopharyngeal and oropharyngeal cavities and produce highly contagious, acute respiratory disease that results in significant morbidity and economic costs. Typical influenza viral infections in humans have a relatively short incubation period of 1 to 2 days, with symptoms that last about a week including an abrupt onset of fever, sore throat, cough, headache, myalgia, and malaise. When a subject is infected with a highly virulent strain of influenza these symptoms can progress rapidly to pneumonia and in some circumstances death. Pandemic outbreaks of highly virulent influenza present a serious risk to human and animal health worldwide.
The immunodominant antigens present on the surface of influenza viruses are hemagglutinin (HA or H) and neuraminidase (N). Genetic reassortment between human and avian influenza viruses can result in a virus with a novel hemagglutinin of avian origin, against which humans lack immunity. In the 20th century, the pandemics of 1918, 1957, and 1968 were the result of such antigenic shifts. The avian influenza outbreaks of the early 21st century caused by H5N1, H7N7, and H9N2 subtype influenza viruses, and their infection of humans have created a new awareness of the pandemic potential of influenza viruses that circulate in domestic poultry. The economic impact of a major influenza pandemic has been estimated to be up to $165 billion in the United States alone, with as many as 200,000 deaths, 730,000 hospitalizations, 42 million outpatient visits, and 50 million additional illnesses.
To combat influenza infection, neuraminidase inhibitors have recently been developed. Clinical studies carried out for the Food and Drug Administration's (FDA) approval of neuraminidase inhibitors in the United States showed that successful treatment primarily depends on prompt treatment after the first clinical symptoms occur. Unfortunately, it is generally not possible for even experienced medical professionals to reliably diagnose influenza solely on the basis of the patient's clinical symptoms because other viruses which infect the nasal or pharyngeal cavity, such as adenoviruses, parainfluenza viruses, or respiratory syncitial viruses (RS viruses), cause similar symptoms. To effectively treat influenza infection it is necessary to begin treatment as early as possible in the development of the infection, ideally upon the onset of non-virally specific clinical symptoms.
A variety of methods have been used to detect influenza viruses clinically. In one example, influenza viruses are detected by culturing samples obtained from a subject on mammalian cells such as Madine-Darby Canine Kidney cells (MDCK). Culturing mammalian cells is costly and time consuming (taking up to 14 days) and is thus not of immediate relevance for the diagnosis of the individual patient. Other methods of detection that have been developed include immunofluorescence assays (IFA), enzyme immunoassays (EIA), and enzyme-linked immunosorbent assays (ELISA) that use antibodies specific to influenza virus antigens. Culture and serological tests require long completion times (5 days to 2 weeks) with potentially greater exposure of technical personnel to infectious agents. Immunoassays are generally faster (30 minutes to 4 hours) but often require substantial sample handling and rely on subjective determination of results by technical personnel. Furthermore, these tests typically are not capable of rapidly differentiating between the influenza types and subtypes, some of which have pandemic potential.
Hence the need remains for a test that provides sensitive, specific detection of influenza virus types and subtypes in a relatively short time, so that diagnosis is completed in sufficient time to permit effective treatment of an infected person.